WWOOF-ing is seriously one of my favorite things to do, and I think New Zealand (with its welcoming, cheerful, and can-do attitude) has been one of my favorite places to do so.
WWOOF-ing is really what you make of it. You purchase a membership for minimal cost (I think NZ cost me about US$20 for a year membership) in order to access the contact info for hosts all around the country, and these hosts range from small subsistence gardens to wineries to sheep farms to orchards, to whatever. The hosts that you first contact don't always have room to take you in during your preferred days, but there are so many options it will be difficult to not have a place to go. During my time there I was able to work in gardens, help clean and repair boats, help shear sheep, pick fruit in an orchard, help my hosts run some of their rafting company tours, dig on the beach for food, work in a vineyard, and much more.
You get to learn so much about the country and the people by living and working with them--often times your hosts will practically adopt you and take you on outings to amazing places that are literally in no guidebooks (I've checked at multiple bookstores) or suggest activities or ways to get around. You also learn a lot about yourself and come away a stronger person for the experience.
If you have the time and want to fully immerse yourself in Kiwi culture, I would highly recommend WWOOF-ing. Once you're there, it's practically free, and you have a decent amount of free time outside of the hours you have to work that you can go exploring.
What would you improve about this program?
I wouldn't change it, but people who do this program shouldn't expect to be coddled, either by WWOOF staff or your hosts. Once you have your membership, it's up to you to make things happen--I'm pretty sure all the WWOOF staff do it keep the membership program running. It's honestly not that hard to make your own way, and it means you get to choose your own adventure. :)